In recent years, changes in world political structure have provided the major oil producing countries with tremendous economic power over the highly industrialized countries which depend to a great extent on them for fuel. Although efforts have been under way for some time to improve oil prospecting and recovery methods and equipment to enable alternative oil sources to be discovered and exploited, the recent political and economic pressures have added a particular sense of urgency to the search.
One approach which has received considerable attention in the past ten or fifteen years is the use of various types of off-shore well-drilling platforms. More particularly, mobile, semi-submersible drilling platforms have been used. These are considered preferable to stationary platforms since the semi-submersibles are not dependent upon the presence of suitable ocean bottom structure for a support foundation, and may be moved rather easily from one drilling site to another. A notable disadvantage of semi-submersible drilling platforms, compared to the stationary type, is their inherent tendency to rise and fall with the sea waves as the waves move past the drilling station. Any relative motion between a drilling platform and the earth is undesirable, of course, since the length of the drill string must somehow accommodate to the changes. Where the changes are large and frequent, this can present serious impediments to continuous drilling.
Modern semi-submersibles are designed to reduce heave, pitch and roll motions through the use of large size and mass supported by submerged buoyant bodies which are coupled to the drilling platform by columns having a relatively small waterplane area. Due to the small waterplane area and large mass of present-day semi-submersibles, passing waves subject the device to only small driving forces which lead to small accelerations and short displacements. The heave motion which remains may frequently be compensated for, at least in part, through the use of drill string heave compensators, as is known in the art.
Unfortunately, there occur sea states which override present-day capabilities to deal with heave and require drilling operations to cease. Daily operation costs for the semi-submersible and its crew continue, however, and may amount to annual losses in the millions of dollars. Various solutions to these problems have been suggested. First, much larger platforms could be used to create a more favorable ratio of platform mass to wave energy. Theoretically, this approach has promise; however, the much higher construction and operation costs and anticipated structural limitations severely limit the amount of additional mass which may be economically provided. Second, it has been proposed to provide active six-degree-of-freedom hydrodynamic controls for known semi-submersibles. This approach may have some promise for future semi-submersibles, but is not clearly applicable to existing platforms or those already planned and being built. Third, a type of more effective passive damping system could be provided, which preferably would be retrofitable to existing semi-submersibles. The present invention concerns a unique type of retrofitable passive damping system.
Present-day semi-submersibles usually include heave damping structure in the form of the flat deck surfaces of the submerged hulls or pontoons. Unfortunately, these surfaces are too small in area and located at too shallow a draft to reduce heave significantly, especially in very heavy weather. To locate the pontoons at greatly increased depths on the type of rigid support legs in use today would create severe structural design problems and render the vessel far less stable in the floating condition. One such prior art system is shown in an article in Ocean Industry, August, 1971 by Jack R. Hilder, Jr. Another prior art system is shown in U.S. Pat. No. 3,673,974, granted to James C. Harper in July, 1972, which discloses a semi-submersible having a perforated mat submerged to a shallow depth for stabilization. The mechanical complexity of these systems, coupled with the substantial difficulty of retrofitting existing platforms, may be responsible for their slow acceptance by the off-shore drilling community.
It is also known in the prior art to provide a flat plate suspended at a shallow draft below the surface of the water and attached to a vessel having a conventional, surface vessel or displacement type hull. As will be shown hereinafter, in order for a submerged plate to have a significant effect in stabilizing the heave motion of a vessel, it is necessary that the area of the submerged plate be a multiple of the waterplane area of the vessel. In surface vessels having displacement-type hulls, the waterplane area is necessarily very large, since obviously all of the vessel's buoyancy is provided by the hull's displacement at and near the water's surface. Consequently, a submerged plate to provide effective heave motion stabilization must have an overall area as much as several times greater than the waterplane area of the vessel. It will be apparent that plates of this size are entirely impractical for displacement-type surface vessels. Where, as with the present invention, the vessel is one having a semi-submersible type hull, the waterplane area is very small compared to a surface vessel of equal mass. This makes it possible to provide a submerged damper plate whose area is many times greater than the waterplane area and yet have the outer dimensions of the submerged plate be, in most instances, no greater than the dimensions of the vessel itself. Thus, the plate can readily be accommodated under the vessel and can even be stored thereunder, in an elevated position, when the vessel is being transported from one site to another.
Typical examples of prior art disclosures wherein submerged plates are applied to surface vessels having displacement-type hulls comprise the Pleva U.S. Pat. No. 1,096,192 and Farquhar U.S. Pat. No. 283,091. In view of the above considerations, the teachings of these patents are not considered as being applicable to the present invention. In the case of the Pleva patent, moreover, the suspended plate is positioned at only a slight distance below the bottom of the boat to which it is attached. Such shallow deployment of the plate is obviously essential in Pleva for the hull to be maintained in a floating condition. More particularly, one can readily visualize that, in Pleva, if the plate were positioned sufficiently below the boat so as to be located in relatively still water, regardless of surface conditions, in the presence of wave action the suspended plate would readily be lowered in the water as the boat itself is lowered in a wave trough. Unfortunately, the plate and the attached boat would be prevented from rising as the crest of a wave approaches, thereby swamping the boat. Accordingly, it is essential, in order for Pleva's disclosure to be operable, that the plate be suspended at a relatively shallow depth. In the case of Farquhar, the damper plate is rigidly suspended beneath the vessel which would cause very high compression loadings in the support and would require the use of impractically heavy structure to sustain the compression loads. Moreover, the area of Farquhar's plate is so small as to provide negligible damping effect.
According to basic wave theory, the vertical component of water wave particle motion decays exponentially with depth such that motion is halved for every increase in depth below the mean water surface of one-ninth of the wave length in the deep ocean. Virtually all motion ceases below 100 meters, even in the most violent storms. Also, it is known that increased wave amplitudes are accompanied by longer ocean wave lengths and periods; whereas, the heave response of conventional semi-submersibles increases markedly as ocean wave periods approach the heave resonance period of the semi-submersible. Thus, conventional semi-submersibles, even those having shallow draft dampers as discussed above, are subject to large heave excursions during sea conditions frequently encountered above known submerged oil fields, such as in the North Sea.